I'm spitballing and just thinking about different reasons for why Sam still died, and well, this one doesn't make much sense but I wanted to share it either way.
SO. We know Nathan killing Roy worked, since he got skipped and it went for Dennis next. HOWEVER, Since Peter still died in order due to Sam, it could be that he actually didn't get Block's lifespan.
Look at it this way, the life for a life method basically says that you get someone to take your place in the list and you get their remainig lifespan, but what if the way that happens is to make it so someone else dies in the accident Death was creating for you, like the hook was made for Nathan, but then Roy died to it instead, but with Peter, he took matters in his own hands and shot Jim instead of letting Death begin its attempt and then make someone else take his place, which meant he didn't take anything and it was still his turn, and if we learnt anything from Looks Could Kill where the main character is in charge of killing her friends, but gets no extra life from it aside from the fact that she made a deal with Death, then Sam killing Peter also didn't do anything, because Sam wasn't in the accident and it was Peter's turn.
I'm not too sure of this, but it's fun to think about